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The Wave That Shook The World

Chat to oceanographer Dr Simon Boxall after the Equinox programme on Monday 24 January at 10pm.

Dr Duncan L Copp

January 2005

The surface of the planet we live on may seem unchanging, yet geological forces powered by the interior heat of Earth, left over from its formation, constantly stress and strain the crust deep beneath our feet. Tsunamis are caused by sudden cataclysmic upheavals in this crust. It was one such upheaval that caused the disastrous waves that struck the Indian Ocean on Boxing Day.

Earth's crust is not a continuous shell, but is broken into approximately eight major sections, called tectonic plates. Great stresses and strains occur at the boundaries of these plates, where they jostle past each other. Here the crust is either being torn apart or is sinking below another section. Periodically, these pent-up stresses release themselves in the sudden shudder of an earthquake or the violent eruption of a volcano. Plate boundaries can be found on the continents and also under the oceans. Should a particularly strong shift occur in the crust along a plate boundary under the ocean, the result will be a powerful earthquake, often followed by a tsunami.

A tsunami is a direct result of the displacement (either up or down) of the ocean crust at an underwater plate boundary, which in turn displaces the water above. The Boxing Day tsunami occurred after the displacement of crust along a 1000-kilometre stretch of the India Plate where the crust is sinking under the Burma Plate.

Tsunamis can also occur when massive landslides slip into the sea, triggered by a volcanic eruption, earthquake, or both. During the eruption of Krakatoa in 1883, large sections of the island collapsed into the Sunda Straights near Java. This displaced a huge amount of water, resulting in a series of tsunamis, the greatest of which was 40 metres high.

Beach waves

Normal waves we see on the beach are formed by the passage of wind over the ocean or lake. The wind pushes and piles up the surface of the water, forming a 'swell'. The size of a swell depends on two main factors: the strength of the wind and the distance the wind has blown over the open water, known as the fetch. Typically, a long fetch and strong winds makes for big swells. The huge rollers loved by surfers in Hawaii and California owe their existence to powerful tropical storms way out in the Pacific.

Swells remain relatively constant while out at sea, but when driven in to shore their characteristics change. As the depth of water decreases, the swell slows down and grows taller, forming a wave. The height a wave can reach, called the amplitude, depends mainly on the distance between two successive wave crests, called the 'wavelength' – the larger the wavelength the greater the amplitude of the wave. Typically, a series of big Californian rollers may have wavelengths of 150 metres or so and amplitudes of a few metres. Eventually, the wave grows so high it becomes unstable, arching over before breaking onto the beach.

The speed at which a wave travels depends on water depth, and how deep the wave's energy extends vertically through the water – the greater the water depth, the faster it can travel. Normally, waves formed by wind don't extend much beyond a few tens of metres below the water. This depth and speed relationship explains why waves slow down when they come into shore; as the water depth decreases, so does the wave's velocity.

Tsunami waves

While the formation of tsunamis is different from normal beach waves, their physical characteristics are the same, only of much greater proportions. The effective depth of a tsunami wave is far greater than a beach wave, since the displacement of crust affects the water above it vertically through the whole depth of the ocean, from the seabed to the sea surface. This can be as much as 4000 metres in the Pacific. As depth affects wave speed, tsunamis can travel at ferocious speeds in the open ocean – up to 830kmh – as fast as a jumbo jet.

After the initial displacement of the crust, a series of tsunamis will ripple outwards in a similar way to the ripples on a pond from a pebble. Their wavelength, however, is much greater than swells caused by wind. There can be hundreds of kilometres between the crests of tsunamis, compared to the tens of metres for wind-produced swells. While out at sea, a tsunami may only be a metre or so high and hardly noticeable to ocean-going ships. But the height radically changes as the tsunami moves towards the land.

The majority of continents are surrounded by a flat shallow shelf typically a 100 metres or so underwater. As the tsunami approaches this 'continental shelf', its speed slows and height builds – in the same way that wind-generated swells do when they meet the shallow water of the seashore. It's here that tsunamis grow into huge walls of water. Owing to their enormous wavelengths, tsunamis can grow many tens of metres in height when they reach the shallow waters.

Why tsunamis are so destructive

When on the move, water is incredibly powerful. Many people get into difficulty while swimming in a river or the sea simply because they underestimate the energy stored in moving water. The energy stored up in a wave is proportional to the square of its height. This simple relationship explains why tsunamis are so destructive.

As an example, a typical fairweather 1-metre high wave delivers as much energy as 10 one-bar electric heaters for every metre of shoreline that it hits. But a 20-metre high tsunami, for example, will deliver 20 x 20 = 400 times as much energy – that's 4000 one-bar heaters. This explains why waves are one of the most destructive forces known to man.

The long wavelengths of tsunamis mean that the crests are typically spread out over greater distances compared to a normal beach wave. The result is that tsunami waves last much longer when they break on the shore.

Added to all this is the weight of water: a cubic metre weighs one tonne. It's the continuous fast-moving and sudden flood of water which is so damaging. The sheer volume of water from multiple tsunami waves travelling at high-speed means little can stand in the way, even with relatively small tsunamis just a few metres high.

The coastline can influence the destructive outcome of a tsunami. The distance a tsunami moves inland is known as the run-up. With wide, flat expanses of beach, the wave energy dissipates relatively quickly. A band of vegetation running along the shore, such as a mangrove swamp, can also greatly reduce the run-up of a tsunami. However, the opposite can occur with inlets and bays. They can act as funnels for tsunamis, restricting and focusing their destructive force.

Predicting tsunamis

What can be done to protect against tsunamis? Unfortunately, predicting events which lead to their formation (earthquakes and volcanic eruptions) is still scientifically challenging. However, following the tsunami that wreaked havoc on the shores of the Hawaiian islands on 1 April 1946, a modest tsunami warning system was installed in the Pacific, mainly to alert US military bases. This has since grown into a much more comprehensive network, known as the Pacific Tsunami Warning System (PTWS). Twenty-four seismic stations dotted throughout the Pacific monitor earthquake activity, measuring their magnitudes which can be used to help calculate the risk of tsunamis along the Pacific coastline.

Unfortunately, no such system exists for the Indian Ocean. Here, powerful earthquakes responsible for causing tsunamis are much rarer. However, the sad fact is that had such a system been in operation on 26 December 2004, coastal districts would have had up to three hours to be evacuated, possibly saving tens of thousands of lives.

While tsunamis are devastating, they are relatively simple to avoid, given adequate evacuation time. The best way to escape the incoming water once a warning has been issued is to head for high ground – this need only be a few tens of metres in elevation. Even better is to try and head inland as far as possible. It's very rare that the run-up of a tsunami reaches for more than a few kilometres.

Without any early warning, the first indication that a tsunami is imminent is often the sea retreating from the shoreline, revealing an unnaturally large expanse of seabed. The water is being drawn up by the tsunami as the crest develops when the wave moves into shallower water. On seeing this, you must act quickly; you may only have a few minutes before the wall of water strikes.

You can read more about the recent tsunami and the global economic isues it has raised on the Channel 4 site After the Tsunami.

Find out more

Channel 4 is not responsible for the content of third party sites

Websites

2004 Indian Ocean Earthquake
http://en.wikipedia.org/wiki/2004_Indian_
Ocean_earthquake#Signs_and_warnings

Excellent and up-to-date information from Wikipedia. Explains how an earthquake caused the tsunami devastation, with an interactive map and a tsunami timeline of key events in the Indian Ocean. Also covers the need to have an early warning system in place, as is currently the case in the Pacific.

Asian disaster – How to help
http://news.bbc.co.uk/1/hi/world
/asia-pacific/4131881.stm

BBC site shows which countries were affected by the disaster, and offers a comprehensive list of agencies asking for help.

Oxfam demands trade relief for countries affected by tsunami
www.oxfam.org.uk/press/releases/
asiaquake_trade_120105.htm

Oxfam argue that EU Trade Commissioner Peter Mandelson should act immediately by removing all tariffs on textiles and clothing imports to the EU from affected countries.

Plate tectonics and people
http://pubs.usgs.gov/publications/text/
tectonics.html#anchor24562816

Good overview of how plate tectonics affect Earth and the people that inhabit it.

The South-East Asia earthquake and Tsunami Blog
http://tsunamihelp.blogspot.com/
Blogs often have more up-to-date news than the usual media outlets so log on here to find out what's being done and what's still left to do. This site has tons of links and information.

Tsunami early warning 'next year'
http://news.bbc.co.uk/2/hi/south_asia/4168917.stm
Unesco is set to give the go-ahead for an Indian Ocean early warning centre. This would involve placing a number of deep water measuring devices on the ocean floor, which would relay wave movements to surface buoys and then to a satellite.

Tsunami Earthquake Appeal – Disasters Emergency Committee
http://www.dec.org.uk/
This is the main British Appeal, which represents key UK agencies like Oxfam, British Red Cross, Save the Children, Action Aid and others. Donate online, via text or phone. Text 'donate' to 83321 and £1.50 will be donated to the appeal (no VAT or charges will be made). Phone 08700 60 60 900.

Tsunami in Asia – UK citizens missing
www.fco.gov.uk/servlet/Front?pagename=
OpenMarket/Xcelerate/ShowPage
&c=Page&cid=1022686957237

The Foreign Office has up-to-date information on those British citizens known to be dead or missing. There is an emergency number for concerned friends and relatives and travel advice on those countries affected.

The Tsunami Page
www.drgeorgepc.com/
Comprehensive pages from a tsunami expert, with FAQs, diagrams, articles and publications and a wealth of information on tsunamis, volcanoes, earthquakes, hurricanes and more.

Books

book cover

Encyclopedia of Earthquakes and Volcanoes edited by David Ritchie and Alexander Gates (Facts on File, 2001)
Explains the terms and concepts associated with seismology and volcanology and covers the places of the world where these destructive phenomena have occurred.
Get this book

 
book cover

Essentials of Geology: Portrait of Earth by Stephen Marshak (W W Norton, 2003)
An introduction to geology that weaves the theory of plate tectonics and the concept of Earth systems science into its narrative.
Get this book

 
book cover

Furious Earth: The science and nature of earthquakes, volcanoes and tsunamis edited by Ellen Prager (McGraw-Hill, 1999)
Brings together top scientists in the field to examine the nature of unpredictable and destructive forces, such as earthquakes, volcanoes and tsunamis. It provides the story and science behind these forces, from Earth's evolution and plate tectonics to disaster warnings.
Get this book

 

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